Delivery of therapeutic fluid agents through the skin requires penetration of the stratus corneum, the outer layer of the epidermis, and the layer of skin which normally provides a largely impervious barrier to the flow of microbes and most other materials into or out of the body. Penetration of the epidermal layer is conventionally accomplished by means of a hollow needle or cannula, beveled at the penetrating end so as to provide a sharp point for local shearing of the skin, both at the surface, and in the course of continued penetration, as the needle is driven down through the epidermal layers into the dermis. Some known methods apply suction around the injecting cannula to distend or engorge the underlying blood vessels, in order more efficiently to achieve intravenous infusion of material. Since the dermis contains live nerve cells, the penetration of the needle is often uncomfortable to a patient.
Among the methods known for introducing drugs or other therapeutic agents into the body, some employ a multiplicity of needles. The known methods employing multiple needles require devices which are difficult to fabricate and therefore costly.
Other methods are known in the art for introducing therapeutic agents into the dermis so that they can be taken up by the circulatory system and distributed within the body to clinical advantage. One such method is simple topical application, such as a patch, which relies upon slow diffusion of the agent through the epidermis. Another method employs a jet injector whereby one or more streams of fluid agent are driven forcibly through the epidermis without further mechanical separation of the outer tissue layer. These methods, reliant upon passage of fluid through the epidermis, provide highly indeterminate and variable rates of diffusion and thus of total quantity of agent introduced. In many applications, this indeterminacy is unacceptable, either because the therapeutic agent is dangerous or costly in quantities exceeding the desired concentration of the substance.